LNG World Shipping

PSVs dominate LNG-fuelled service and supply ships

LNG-fuelled harbour vessel Econuri is deployed at Incheon in South Korea

In this second of our four sector-specific guides to the world's LNG-fuelled tonnage, Mike Corkhill reports on growth in bookings for PSVs

There has been little change in the number of service and supply vessels in the world LNG-fuelled fleet over the past year. According to LNG World Shipping’s new review of gas-powered ships that are not LNG carriers, both the operational and on-order fleets of service and supply vessels rose by three units.

Platform supply vessels (PSVs) figure prominently in the at-sea LNG-powered service and supply fleet, accounting for 20 of the 33-ship complement. The orderbook, in contrast, shows much more variety, with a portfolio encompassing dredgers, a jack-up rig, a semi-submersible crane vessel, a cable-layer and a wind farm installation ship.

DEME of Belgium has been a leading advocate of LNG-fuelled dredgers and has investigated the challenges of using gas to power them. Factors to be weighed up include bunker tank size and location, LNG bunker availability, shipboard bunkering connections, crew training and the step load capability of the various gas-burning engine options.

Combined with its cold box encasement a cylindrical Type C LNG bunker tank occupies about three times as much space as a prism-shaped tank of marine gas oil (MGO) containing the same energy.

While Type C pressure vessel tanks offer many advantages, accommodating them has a significant impact on vessel layout. Also, the tank location restrictions imposed by the new International Code of Safety for Ships using Gases or other Low-Flashpoint Fuels (IGF Code) add another level of complexity to the design challenge.

Naval architects working on the DEME dredgers found that dual-fuel diesel engines worked best for the vessels as they have a better step load capability than gas engines. The mix of LNG and MGO bunker tank capacities associated with such propulsion units also offers a measure of redundancy.

The aim is to optimise the use of LNG, assuming that the cost of this fuel is lower than MGO, but to provide sufficient MGO as a backup. The crew can switch to MGO if the dredger has to operate longer than planned before the next LNG bunkering stop.

Harbour tugs are the second most popular type of LNG-fuelled service and supply vessel, after PSVs. There are now six gas-powered tugs in service and eight on order. Wärtsilä, with its medium-speed dual-fuel engines, and Rolls-Royce, with its gas-only units, have both enjoyed success in providing tug propulsion systems.

LNG-fuelled tugs are in operation in Norway, China and Japan, while Dubai and Singapore are poised to join the club as a result of vessels on order. The next new member could well be the US where a number of designs for gas-fuelled tugs and towboats have been tabled.

Three of the tugs on order are under construction at the Gondan yard in Spain for Østensjø Rederi AS. The owner has opted for Wärtsilä engines and the series is slated to provide ship assist and escort services for the LNG carriers loading cargoes at the Statoil export terminal near Hammerfest in northern Norway.